Variable tuning system



Nov. 27, 1945. B, STOTT 2,390,009

VARIABLE TUNING SYSTEM Filed May 25, 1944 INVENTOR I A/Maza frorr.

/ +6 BY g/un/M/ ATTORNEY Patented Nov. 27, 1945 VARIABLE TUNING SYSTEMHaroldv B; Stott, Glen Olden, Pa., assignor to Radio Corporation ofAmerica, a corporation of Delaware I Application May 25, 1944, SerialNo. 537,277

Claims.

The present invention. relates to variable tunnlng systems for radiosignal receiving apparatus and. the like, and more particularly to avariable tuning system for the signal input and oscillator circuits of asuperheterodyne receiver.

A primary object of the invention is to provide a compact and simplifiedtuning system for the above-mentioned circuits in a. single unit,whereby the. system is adapted for use in miniature portable receiversof the personal or pocket type.

A further object of this invention is to provide a unitary, relativelywide range, variable tuning system for the. signalinput andoscillatorcircuits oil a superheterodyne receiver which includes variable capacitytuning of both the oscillator'circuit and the signal input or antennacircuit with unitary tuning means adapted to occupy minimum space in areceiver.

Another object of this invention is to provide a coaxial arrangement forthe variable tuning capacitors in order to provide a compact unitarytuning system for high frequency signal circuits. Astill. further objectis to provide a tuning arrangement which employs a pair of coaxial.

relatively movable .cylindrical members, one of which has formed thereona pair of metallized films constituting respectively one of theelectrodesof the variable capacitors which are adapted to tune thesignal and Oscillator circuits, the

I an improvement over the coaxial tuner described other cylindricalmemberconstituting a common electrode which is connected to ground andforms theother electrodes of said variable capacitors.

A still further object is to provide a coaxial capacitor tuner of thetype mentioned in which the metallized film constituting the electrodefor the oscillator tuning capacitor is so shaped that there is obtainedsubstantially exact tracking between the tunable circuits throughout thetuning-range-of the receiver.

The novel features characteristic of my invention are-setforthwithparticularity in the appended claims. The invention itself, however,both as to its organization and mode of operation together with furtherobjects and advantages thereof. will best be understood by reference toY Fix. 2: is an end view ofthe movable tuning ele- V I mentxas viewedfrom the left in Fig. 1; t Fig: Bis-a cross-sectional view ofthe movabletuning, element taken onthe line 3-3 of Fig. "2";

and claimed in U. S. Patent N 0. 2,338,134 to Sands and Daley, assignedto the same assignee as the present application, and differs therefromin that both the signal and oscillator circuits are capacity tuned sothat an ordinary solenoid 0r universal wound oscillator coil may beused. As a consequence, the need of a variable pitch oscillator coilnecessary in a permeability tuned system is eliminated, tracking of thesignal and oscillator circuits in the present application beingaccomplished by properly shaping the variable plate electrode of thecapacitor fo the oscillator along its length in the direction ofmovement with respect to the stationary plate electrode. Also,

by eliminating the movable magnetic core which effects the tuning of theoscillator coil, the space normally occupied by said core, that is, thecenter line area of the coaxial cylinders may be occupied by the drivemechanism which in the present case is constituted by an axiallydisposed screw member.

Referring to Fig. 1, a frame member I adapted to be supported on thereceiver chassis or otherwise is provided with a pair of upright arms 2,3 which carry respectively a bearing 4 and a bearing bracket 5. A screwshaft 6 is adapted to have its opposite ends J'ournalled fo rotationwithin said bearings, the shaft being extended at the left to providefor manipulation of the shaft, such as by means of a manual controlknob, not shown, or other suitable means.

Concentrically arranged with respect. to the drive shaft and adapted foraxial movement is a cylindrical member 1 of insulating material, such asceramic, glass or Lucite. Formed on the inner surface of the movablemember 1, by fusing, electro-deposition or other process, are two metalfilms or coatings 8 and 9 (Fig. 2) which constitute the movable platesof the capacitors which serve to tune the oscillator and signalcircuits, respectively. Flexible leads 8 and 9' serve to connect themovable capacitor plates 8 and 9 to their respective circuit parts ofthe receiver. The films 'orplates need be only about .002 or .003 inchl8 of fibre or Bakelite.

thick and should extend through an are somewhat lessthan 180 degrees inorder to provide proper isolation between the two circuits. Theoscillator condenser plate 8 is suitably shaped (Fig. 3), tapering alongits length from the leading portion Ill, to permit the oscillatorcircuit to track with the signal circuit over the tuning range of thereceiver. Shaping of the plate is accomplished by masking. With thismethod of tracking, the series padding condenser normally used to obtaintracking in capacity tuned super heterodyne receivers is eliminated.

The means for imparting axial movement to the cylindrical member I .is a.nut II which is threaded on the screw shaft 6 and which is affixed bysuitable means, such as a spider l2, to the cylindrical member at theleft end thereof. A part l3 depending from the member 1 and arranged forsliding movement within an elongated slot I4 in the frame I prevents themember I from rotating during its axial travel.

The cylindrical member I'is adapted, in its axial movement from left toright, to encompass va ying amounts ofthe surface area of a stationarycylindrical member I5 which serves as the common plate for the twocapacitor metallic films or plates 8 and 9. As shown by thecross-sectional view 'of Fig. 4, the stationary or inner cylindricalmember comprises a thin metallic sleeve 1 6 which may be of steel. Theedge ofthe sleeve at the left is turned down for receiving a grommet I1which is carried at one end of an insulating tube The other end of thetube through which the screw shaft is adapted to pass is supported by asecond grommet l9 carried by the right-hand arm 3. The grommet i9 isspanned by the bearing bracket 5 and a brass collar 6' attached to theright-hand end of the drive screw is adapted to run free in an aperture5' ofthe bearing bracket. In addition'to serving as a support for thecommon stator plate or stationary member IS the tube l8 serves as thecoil 26 form o support for a coil which is included in the oscillatorportion of the system.

The metal sleeve I6 is connected to frame I (or ground) by means of alead I6 and is provided on its outer surface with a coating 2| ofinsulating material, such as enamel, in order to insulate the movablecapacitor elements 8 or 9 from the stationary capacitor element 16.Preferably, the coating 2| should be of relatively high dielectricconstant to provide suflicient capacity for tuning the circuits througha'desired frequency range.

circuit of a superheterodyne receiver. The converter tube is shown at25. The signal control grid of the tube is indicated at 26 and isconnected to a signal input circuit 21 comprising a loop antenna orinductance 28 and the shunt tuning capacitor constituted by the fixedgrounded electrode l6 and the movable electrode 9. The oscillator, gridis indicated at 29 and is connected to an oscillator circuit 30comprising the coil .26 and the shunt tuning capacitor constituted bythe fixed grounded electrode I 6 and the shaped movable electrode 8. Thecoil 3|, connected to grid 32 which acts as an anodeof the oscillatorsection of the tube, is coupled to the grid coil 20 and providesfeedback for the oscillator, and intermediate frequency signals are.derivedthrough the output anode circuit 33 and a suitably-tunedintermediate frequency coupling transformer 34. As the converter circuitis shown only for the purpose ofillustrating a present preferred-use ofthe coaxial tuner, further description of the circuit is believedunnecessary. v While I have shown and described a preferred embodimentof my invention, it will be understood that various modifications andchanges will occur to those skilled in the art Without departing fromthe spirit and scope of thisinvention. I therefore contemplate by theappended'claims to cover any such modifications as fall, within the truespirit and scope of my invention;

WhatIclaimis: e l. A tuning system for a radioreceiver of. thesuperheterodyne type having asignal input circuit and an oscillatorcircuit, a pair of variable capacitors included one in each of saidcircuits, a

fixed cylindrical member constituting an .electrode which is common toeach of said pair of variable capacitors, a second cylindrical membercoaxially arranged and movable with respect ,to

said fixed member, and a pair ofarcuate plates formed on said'mo'vablemember, one of said plates constituting the variable electrodeiofthesignal circuit capacitor and the other plate constituting the variableelectrode of the oscillator circuit capacitor, said latter plate beingshaped Disposed within the metallic sleeve 16 is a cyl- 56 to provide apred rmi ac g relation b inder 22 of magnetite which is adapted toconfine the field about the oscillator coil 26 to thereby improve its Q.Styrol tape or other insulating material indicated at 23 iswrapped'around the magnetite cylinder in order to insulate it fromground, and interposed between the layer of insulating tap 23 and theinner surface of the sleeve I6 is a layer 24 of copper which maybe ofthe order of .005 inch thick in order to 'keep the oscillator field outof the metal sleeve to thereby further improve the coil Q. In order thatthe drive screw 6 which is disposed in concentric relation with the coilmay not have a deleterious effect on the coil Q it should be of highconductivity metal or plated with such metal.

In Fig. 5 there is disclosed a modified form of coaxial tuner in thatthe inner or stationary cylindrical member is the same as in Fig. 4except that the enamelled dielectric coating 2! is y omitted, and theouter cylindrical member I i the other plate constituting the variableelectrode cuit and an oscillator circuit, a pair of variable capacitorsincluded one in each of said circuits, a fixed cylindrical memberconstituting a common electrode of said pair of variable capacitorsasecond cylindrical member co-axially arranged and movable withrespecttosaid fixed member, a pair of'arcuate platesformed on saidmovable member, one of said plates constituting the' variable electrodeof the signal circuit capacitor'and of the oscillator circuit capacitor,and a screwthreaded shaft for effecting movement of the secondcylindrical member with respect to the first member.

4. A tuning system as defined in claim 3 wherein the oscillator circuitincludes a coil which is mounted within the fixed cylindrical member,and the inner surface of the fixed cylindrical member is lined withmagnetite in order to improve the Q of the oscillator coil.

5. A tuning system as defined in claim 3 wherein the oscillator circuitincludes a coil which is wound around an insulating coil formconcentrically arranged within the fixed cylindrical member, and thescrew shaft is extended through the coil form without seriouslyaffecting the Q of the coil.

HAROLD B. STOTT.

